This invention relates to development systems for electrostatic processors and, more particularly, to crossmixers for such systems.
In a conventional electrostatic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography," a uniformly charged photoreceptor is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the application of a finely divided, resinous material, called "toner." As is known, that process has enjoyed outstanding commercial success, especially in plain paper copiers and duplicators. Nevertheless, substantial effort and expense are still being devoted to the perfection of the process, including the development step.
The vehicle normally used in electrostatic processors to deliver the toner is a multi-component developer comprising toner particles and relatively coarse "carrier" particles. The toner and carrier (or sometimes carrier coating) are formed from materials which are removed from each other in the triboelectric series, thereby enabling a triboelectric charging process to be employed to induce electrical charges of opposite polarities on the toner and carrier particles. The polarity of the charge for the toner particles is selected to oppose the charge of the latent image so that there are competing electrostatic forces acting on those particles. Specifically, the toner particles at least initially tend to be attracted to the carrier particles, but are subject to being electrostatically stripped therefrom whenever the developer is brought into the immediate proximity of or actual contact with an image bearing photoconductor.
Provision is commonly made in existing development systems for adding additional toner to the developer from time-to-time so that the toner concentration remains at a suitable high level. Additionally, there are passive and active crossmixers for maintaining a more or less uniform distribution of toner throughout the supply of developer so that the developer may be recirculated numerous times without a marked reduction in the quality of the copies produced.
Active crossmixers are externally powered, rather than being wholly dependent on gravity. Consequently, they have several advantages over passive ones. For example, they tend to be (1) better suited to use in compact development systems, (2) less sensitive to variations in the developer charge and (3) at least potentially more effective in (a) blending the toner and carrier particles, (b) reducing the incidents of toner impaction and (c) promoting the triboelectric charging of the toner and carrier particles. Conventional crossmixers of this type have not, however, met with complete success. The primary reason for that is that the usual practice of forming an active crossmixer by fully submerging one or more rotatably driven augers in the developer sump means that substantial input power is required to drive the crossmixer and also creates the risk that significant amounts of developer will bypass the crossmixers.